Answer:
They were made out of painted plaster for three years during World War II because of a metal shortage.
Here is a quote from the Oscars website:
"The statuettes presented at the initial ceremonies were gold-plated solid bronze. Within a few years the bronze was abandoned in favor of britannia metal, a pewter-like alloy which is then plated in copper, nickel silver, and finally, 24-karat gold. Due to a metal shortage during World War II, OscarsĀ® were made of painted plaster for three years. Following the war, the Academy invited recipients to redeem the plaster figures for gold-plated metal ones."